Sloan Digital Sky Survey: Mapping the Universe

The 142-Megapixel Digital Camera

FIVE-COLOR IMAGES—Light energy travels as a wave of varying lengths. Each wavelength of light has characteristic properties, one being its color. The entire spectrum of visible light wavelengths runs from violet (short wavelengths) to red (long wavelengths). Sloan’s digital camera can detect two different wavelength ranges of visible light (red and green) and three ranges of light not visible to the eye: (ultraviolet, which has shorter wavelengths than violet; infrared, which has longer wavelengths than red; and farther infrared, even longer). Because stars, galaxies, brown dwarfs, and quasars give off light at different wavelengths, Sloan’s breadth of color detection allows it to capture a motley crew of space objects.

SUPERIOR SPECTROGRAPHS—Knowing what general color range of light a galaxy or quasar emits allows astronomers to identify what it is, but not much more. A detailed version of that spectrum can tell astronomers what elements the galaxy is made of, how far away it is, and other properties. “On the Sloan telescope, additional instruments called spectrographs break the light up into not five colors, but several thousand colors,” explains Gunn.

To get precise spectra for objects in an interesting area of sky, a one-meter-diameter aluminum disk called a plug plate is precision-drilled with 640 holes. Each hole corresponds to a point of light from a digital picture. A single optical fiber is inserted by hand into each hole, and then the plug plate is mounted underneath the barrel of the telescope. “The fibers must be positioned exactly because each galaxy’s light falls in a given place, and that fiber has to channel it to the spectrographs,” says Gunn. The telescope’s two spectrographs divide each galaxy’s light into several thousand precise wavelengths. Spectrographs on most other telescopes can measure one object at a time, but Sloan’s do 640 at once.

A plug plate is no longer useful after it the spectrum is taken. The Sloan Digital Sky Survey has stockpiled several thousand plug plates and is tossing around ideas for what do with them. (Coffee tables are a popular suggestion.) Scientists are still unsure of what will become of the Sloan telescope itself after its nine-year photo shoot wraps up in 2008. “The telescope and instruments, especially the spectrographs, are still very much state-of-the-art,” says Gunn. “It is almost certain that some project will go forward with some or all of the equipment.” One possibility is to outfit the telescope with a new type of spectrograph in order to survey arrestingly bright stars for the presence of planets. In the meantime, Sloan’s wide-field pictures of dazzling stars and galaxies continue to turn up other rare cosmological jewels.

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